Which protocol is a Cisco proprietary protocol used for trunking switches?

Inter-Switch Link (ISL) is a proprietary protocol used for trunking of switches. If you need to connect non-Cisco switches to a Cisco switch, you must use 802.1Q, the IEEE standard. VTP is not a trunking protocol; it assists in populating VLANs across Cisco switches for conformity and ease of configuration. Cisco Discovery Protocol (CDP) is not a trunking protocol either; it negotiates power by communicating its capabilities with neighboring devices. It also allows for neighbor discovery, but CDP is proprietary to Cisco, so only Cisco devices can communicate.